The Development of Sensitive Diagnostics and Their Use in Shock Tube Studies of Formaldehyde and Formyl Reaction Rate Coefficients

Stanford University, Stanford CA

Investigators

Abstract

PROJECT SUMMARY This is a two-part research program consisting of the development of a new tunable ultraviolet-laser absorption diagnostic and the application of this diagnostic to the investigation of formaldehyde oxidation and nitrogen-oxide kinetics. A sensitive formyl (HCO) absorption diagnostic using a ring-dye laser with an external frequency doubler is developed that takes advantage of the strong ultraviolet (UV) HCO absorption feature near 230 nm. This method should yield a threefold increase in HCO sensitivity over the current laser absorption measurements made at 614 nm. Also used are a sensitive (ppm level) hydroxyl (OH) laser absorption diagnostic at 306 nm and a formaldehyde (CH2O) lamp absorption diagnostic at 174 nm that have been developed previously. Hydroxyl is an important marker of radical pool development in combustion systems and this procedure will enable better isolation of certain reactions in the complex CH2O/HCO/O2 kinetic system. These diagnostics permit measurement of the following reactions over the approximate temperature range 1000 to 1600 K: HCO + O2 ? CO + HO2 (a) HCO + NO ? CO + HNO (b) CH2O + O2 ? HCO + HO2 (c) CH2O + OH ? HCO + H2O (d) Reactions involving formaldehyde (CH2O) and formyl radical (HCO) lie on the primary oxidation pathway for methane and other hydrocarbon fuels. Even with this primary role, there are still large uncertainties in the high-temperature rate coefficients of many of the key reactions in this sub-mechanism. The uncertainty in the experimental measurements of these reaction rates stems largely from an inability to measure important reaction species, such as HCO, with sufficient sensitivity and to kinetically isolate the individual reactions from interfering reactions. Higher-sensitivity measurements of HCO combined with the use of complementary diagnostics for OH permit the design of simpler, more direct experiments with fewer interfering reactions, and hence enable more reliable determinations of rate coefficients for several important elementary reactions. Broader impact This work contributes significantly to graduate education and scientific databases and impacts combustion-chemistry modeling activities in government laboratories and industry. The program provides an opportunity for graduate students to learn state-of-the-art laser and shock-tube methods and gain practical experience in a world-class laboratory. Experimental techniques developed in this program are included in graduate laboratory classes and textbooks. Data, results and expertise acquired in this program are actively shared with government and industry, particularly through the development and distribution of web-based kinetics databases, reviews of combustion-related reaction rates, and personal contacts.